Method for grounding a heat sink in thermal contact with an electronic component using a grounding spring having multiple-jointed spring fingers

ABSTRACT

A grounding spring for electromagnetic interference (EMI) suppression is interposed between a heat sink and a printed circuit board (PCB). The grounding spring comprises a conductive material having an opening formed at its base through which the heat sink makes thermal contact with an electronic module mounted on the PCB. The base makes electrical contact with a peripheral surface of the heat sink, and multiple-jointed spring fingers extend from the base to make electrical contact with conductive pads on the PCB. During compression, the movement of each spring finger&#39;s tip is substantially limited to the z-axis. Accordingly, the final installed location of the tip can be precisely controlled even when the grounding spring must accommodate a wide variety of installed heights of the heat sink relative to the PCB. Preferably, the spring fingers terminate with a concave tip that is less susceptible to sliding off the conductive pads.

CROSS-REFERENCE TO RELATED APPLICATION

This patent application is a continuation application of U.S. patentapplication Ser. No. 11/266,743, filed Nov. 3, 2005, entitled “METHODAND APPARATUS FOR GROUNDING A HEAT SINK IN THERMAL CONTACT WITH ANELECTRONIC COMPONENT USING A GROUNDING SPRING HAVING MULTIPLE-JOINTEDSPRING FINGERS”, which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates in general to the field of electronicpackaging. More particularly, the present invention relates toelectronic packaging that grounds a heat sink used to remove heat froman electronic component.

2. Background Art

Electronic components, such a microprocessors and integrated circuits,must operate within certain specified temperature ranges to performefficiently. Excessive heat degrades electronic component performance,reliability, life expectancy, and can even cause failure. Heat sinks arewidely used for controlling excessive heat. Typically, heat sinks areformed with fins, pins or other similar structures to increase thesurface area of the heat sink and thereby enhance heat dissipation asair passes over the heat sink. In addition, it is not uncommon for heatsinks to contain high performance structures, such as vapor chambersand/or heat pipes, to further enhance heat transfer. Heat sinks aretypically formed of metals, such as copper or aluminum.

Electronic components are generally packaged using electronic packages(i.e., modules) that include a module substrate to which the electroniccomponent is electronically connected. In some cases, the moduleincludes a cap (i.e., a capped module) which seals the electroniccomponent within the module. In other cases, the module does not includea cap (i.e., a bare die module). In the case of a capped module, a heatsink is typically attached with a thermal interface between a bottomsurface of the heat sink and a top surface of the cap, and anotherthermal interface between a bottom surface of the cap and a top surfaceof the electronic component. In the case of a bare die module, a heatsink is typically attached with a thermal interface between a bottomsurface of the heat sink and a top surface of the electronic component.Heat sinks are attached to modules using a variety of attachmentmechanisms, such as adhesives, clips, clamps, screws, bolts, barbedpush-pins, load posts, and the like.

In addition to generating heat, electronic components also generateelectromagnetic radiation. The electromagnetic radiation emitted byelectronic components can cause electromagnetic interference (EMI) ornoise in neighboring electronic components and systems. Regulatorybodies in the U.S. and in other countries set forth regulations thatspecify limits for EMI caused by electronic products. Because of theseregulations and the negative effects of EMI, it is desirable to designelectronic products so that EMI is suppressed (i.e., electromagneticradiation generated within electronic products is minimized oreffectively contained). EMI suppression is extremely important when anEMI producing electronic component is coupled to a heat sink. The heatsink acts as an antenna to further radiate the electromagnetic radiationgenerated by the electronic component.

It is not uncommon for a heat sink coupled to an EMI producingelectronic component to be grounded for EMI suppression. For example, agrounding spring having an aperture that surrounds the EMI producingelectronic component may electronically couple the heat sink and to aground plane of a printed circuit board (PCB) on which the electroniccomponent is mounted.

FIG. 1 illustrates, in exploded view, an exemplary heat sink assembly100. A heat sink 102 includes a central surface 104, which is to beplaced in thermal contact with a module having an EMI producingelectronic component, such as a microprocessor. Heat sink 102 alsoincludes a peripheral surface 106 on which is mounted a grounding spring110. Grounding spring 110 includes a central aperture 112, through whichthe module makes thermal contact with central surface 104 of heat sink102. Grounding spring is made of an elastic and electrically conductivematerial. Four screws 120 extend through through-holes in groundingspring 110 and are threaded into threaded-holes in heat sink 102, tothereby secure and electrically connect grounding spring 110 to heatsink 102.

Eight spring fingers 114 extend from grounding spring 110. The springfingers 114 are formed by cutting and bending grounding spring 110. Twospring fingers 114 extend from each side of aperture 112 so that themodule will be surrounded thereby. The assembly 100 is attached to themodule by dispensing a thermally conductive adhesive between the top ofsurface of the module and central surface 104 of heat sink 102. Whenheat sink assembly 100 is attached to the module, the tips of springfingers 114 contact solder pads on the surface of the PCB on which themodule is mounted. The solder pads are electrically connected to aground plane of the PCB, and thus heat sink 102 is grounded theretothrough the grounding spring 110.

Conventional grounding springs with single-jointed spring fingers, suchas grounding spring 110 having single-jointed spring fingers 114,exhibit a number of disadvantages. Spring fingers 114, which are fragilebecause their length is long relative to their width, can be damagedduring handling of heat sink assembly 100. In addition, the relativelylong length of spring fingers 114 makes them susceptible to sliding offthe solder pads on the surface of the PCB on which the module ismounted, both during and subsequent to the card assembly process (i.e.,the process by which the heat sink assembly 100 is installed onto thePCB).

Another disadvantage of conventional grounding springs withsingle-jointed spring fingers is shown in FIG. 2, which illustrates, inperspective view, a portion of heat sink assembly 100 and a portion of aPCB 210 where the tip of one spring finger 114 engages PCB 210. Asmentioned earlier with respect to FIG. 1, spring finger 114 extends fromgrounding spring 110, which is attached to peripheral surface 106 ofheat sink 102. Typically, spring fingers 114 are relatively long toaccommodate a range of installed heights of heat sink 102 relative toPCB 210. For example, the height at which heat sink 102 is installedrelative to PCB 210 may differ from card assembly to card assembly dueto mechanical production tolerances and the like. Unfortunately, as theinstalled height of heat sink 102 varies relative to PCB 210, theinstalled location of the tip of each spring finger 114 slides along thesurface of PCB 210. Accordingly, it is difficult to precisely controlthe final installed location of the tips of spring fingers 114. As shownin FIG. 2, the final installed location of the tip of each spring finger114 relative to PCB 210 will generally fall along an arc 212 (i.e., thelength of spring finger 114 forms the radius of the arc). The longer thelength of spring fingers 114, the more difficult it is to control thefinal installed location of the tips thereof.

It should therefore be apparent that a need exists for an enhancedmechanism for grounding a heat sink.

SUMMARY OF THE INVENTION

According to the preferred embodiments of the present invention, agrounding spring for electromagnetic interference (EMI) suppression isinterposed between a heat sink and a printed circuit board (PCB). Thegrounding spring includes a plurality of multiple-jointed springfingers, i.e., each spring finger has two or more joints. The groundingspring comprises a conductive material having an opening formed in agenerally planar base portion thereof through which the heat sink makesthermal contact with an electronic module mounted on the PCB. The baseportion of the grounding spring makes electrical contact with aperipheral surface of the heat sink, and the multiple-jointed springfingers extend from the base portion of the grounding spring to makeelectrical contact with conductive pads on the PCB. Eachmultiple-jointed spring finger is configured so that movement of a tipthereof is substantially limited to the z-axis during compression.Accordingly, the final installed location of the tip can be preciselycontrolled even when the grounding spring must accommodate a widevariety of installed heights of the heat sink relative to the PCB.Preferably, the spring fingers terminate with a concave tip that makesit less susceptible to sliding off the conductive pads.

The foregoing and other features and advantages of the present inventionwill be apparent from the following more particular description of thepreferred embodiments of the present invention, as illustrated in theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred exemplary embodiments of the present invention willhereinafter be described in conjunction with the appended drawings,where like designations denote like elements.

FIG. 1 is a perspective exploded view of a conventional heat sinkassembly having a grounding spring with single-jointed spring fingers.

FIG. 2 is a partial, perspective assembled view of the conventional heatsink assembly shown in FIG. 1 in an area where a single-jointed springfinger makes contact with a printed circuit board (PCB).

FIG. 3 is a perspective exploded view of a heat sink assembly having agrounding spring with double-jointed spring fingers according to thepreferred embodiments of the present invention.

FIG. 4 is a partial, perspective enlarged view of the grounding springshown in FIG. 3 in an area of a double-jointed spring finger accordingto the preferred embodiments of the present invention.

FIG. 5 is a partial, perspective assembled view of the heat sinkassembly shown in FIG. 3 in an area where a double-jointed spring fingermakes contact with a PCB.

FIG. 6 is side view of a circuit card assembly having a grounding springwith double-jointed spring fingers interposed between a heat sink and aPCB according to the preferred embodiments of the present invention.

FIG. 7 is a flow diagram of a method for grounding a heat sink inthermal contact with an electronic module using a grounding spring withmultiple-jointed spring fingers according to the preferred embodimentsof the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

1.0 Overview

In accordance with the preferred embodiments of the present invention, agrounding spring for electromagnetic interference (EMI) suppression isinterposed between a heat sink and a printed circuit board (PCB). Thegrounding spring includes a plurality of multiple-jointed springfingers, i.e., each spring finger has two or more joints. The groundingspring comprises a conductive material having an opening formed in agenerally planar base portion thereof through which the heat sink makesthermal contact with an electronic module mounted on the PCB. The baseportion of the grounding spring makes electrical contact with aperipheral surface of the heat sink, and the multiple-jointed springfingers extend from the base portion of the grounding spring to makeelectrical contact with conductive pads on the PCB. Eachmultiple-jointed spring finger is configured so that movement of a tipthereof is substantially limited to the z-axis during compression.Accordingly, the final installed location of the tip can be preciselycontrolled even when the grounding spring must accommodate a widevariety of installed heights of the heat sink relative to the PCB.Preferably, the spring fingers terminate with a concave tip that makesit less susceptible to sliding off the conductive pads.

2.0 Detailed Description

With reference to the figures and in particular FIG. 3, there isdepicted, in exploded view, a heat sink assembly 300 having a groundingspring 302 with double-jointed spring fingers 304 in accordance with thepreferred embodiments of the present invention. The spring fingers 304shown in FIG. 3 are “double-jointed” in that they have two joints. Itshould be appreciated, however, that the present invention is notlimited to double-jointed spring fingers as shown FIG. 3. In accordancewith the preferred embodiments of the present invention, the springfingers may have any multiple-jointed configuration.

A heat sink 310 includes a central surface 312, which is to be placed inthermal contact with a module having an electromagnetic interference(EMI) producing electronic component, such as a microprocessor. Heatsink 310 also includes a peripheral surface 314 on which is mountedgrounding spring 302. Grounding spring 302 includes a central aperture306, through which the module makes thermal contact with central surface312 of heat sink 310. Preferably, four screws 320 extend throughthrough-holes in grounding spring 302 and are threaded intothreaded-holes in heat sink 310, to thereby secure and electricallyconnect grounding spring 302 to heat sink 310. Any number or arrangementof screws may be used, or, alternatively, other attachment mechanisms(e.g., adhesives, welds, clips, clamps, bolts, barbed push-pins, loadposts, and the like) may be used to secure grounding spring 302 betweenheat sink 310 and the printed circuit board (PCB) on which the module ismounted. For example, grounding spring 302 may be secured between heatsink 310 and the PCB by barbed push-pins that pass through through-holesin heat sink 310, grounding spring 302 and the PCB. In anotheralternative, grounding spring 302 may be secured between heat sink 310and the PCB by load posts projecting from heat sink 310 and passingthrough through-holes in grounding spring 302, the PCB, a backsidebolster, and a loading spring. Numerous other attachment mechanismsknown in the art may be used. Those skilled in the art will appreciatethat the spirit and scope of the present invention is not limited to anyone attachment mechanism.

As illustrated, peripheral surface 314 of heat sink 310 is preferablyrecessed relative to central surface 312 of heat sink 310 to provideclearance for the heads of screws 320. However, those skilled in the artwill appreciate that the mechanisms and apparatus of the preferredembodiments of the present invention apply equally regardless of theconfiguration of the heat sink.

Heat sink 310 is formed of a metal, such as aluminum, copper, or thelike. As is typical, heat sink 310 is formed with fins, pins or othersimilar structures to increase the surface area and thereby enhance heatdissipation as air passes over the heat sink. In addition, heat sink 310may contain high performance structures, such as vapor chambers and/orheat pipes, to further enhance heat transfer.

Grounding spring 302 is made of an elastically deformable andelectrically conductive material, such as stainless steel, berylliumcopper, phosphor bronze, hardened steel, spring steel, and the like.Preferably, grounding spring 302 is cut and bent from a sheet of 0.004″half-hard “301” stainless steel.

Preferably, eight double-jointed spring fingers 304 extend fromgrounding spring 302. Two double-jointed spring fingers 304 preferablyextend form each side of central aperture 306 in grounding spring 302 sothat the module will be surrounded thereby. However, any number orarrangement of double-jointed spring fingers 304 may be used. When heatsink assembly 300 is attached to the module, the tips of double-jointedspring fingers 304 contact conductive pads on the surface of the PCB onwhich the module is mounted. The conductive pads are electricallyconnected to a ground plane of the PCB, and thus heat sink 310 isgrounded thereto through the grounding spring 302.

The heat sink assembly 300 is preferably attached to the module bydispensing a conventional thermally conductive adhesive between the topsurface of the module and central surface 312 of heat sink 310. However,a thermally conductive adhesive need not be used. Instead, a thermallyconductive interface such as a thermal gel, grease, paste, oil, pad orother high thermal conductivity material may be interposed between heatsink 310 and the module in the case where another attachment mechanismis used in lieu of thermally conductive adhesive to secure heat sink 310relative to the PCB on which the module is mounted. For example, heatsink 310 may be secured relative to the PCB using barbed push-pins orload posts attachment mechanisms, as discussed above. However, becauseof the additional labor and parts required to utilize the otherattachment mechanisms, electronics device manufacturers often preferutilizing adhesive-mounted heat sinks in order to minimize productioncosts.

FIG. 4 illustrates, in a partial, perspective enlarged view, groundingspring 302 in an area of one of double-jointed spring fingers 304.Double-jointed spring finger 304 includes a first v-shaped joint 402connecting a generally planar base portion 404 to a central fingerportion 406, and a second v-shaped joint 408 connecting central fingerportion 406 to a distal finger portion 410. In addition, distal fingerportion 410 preferably includes a concave finger tip 416 extending in adirection substantially parallel to base portion 404. Preferably, baseportion 404, central finger portion 406, distal portion 410, and concavefinger tip 416 are cut and bent from a single piece of sheet metal toform a one-piece unit of unitary construction. For example, the baseportion 404 may include a cut-out 412 from which central finger portion406 is formed. Similarly, central finger portion 406 may include acut-out 414 from which distal finger portion 410 and concave finger tip416 are formed. The cut-out may be in the form of a hole between twolegs as illustrated in FIG. 4, or, alternatively, the cut-out may be inthe form of a notch adjacent to a single leg. In another alternative,double-jointed spring fingers 304 may be separately formed from baseportion 404 and subsequently attached by a conventional fastening meanssuch as welds, rivets, etc. In yet another alternative, double-jointedspring fingers 304 may be directly attached as individual units to theheat sink.

Preferably, distal finger portion 410 and concave finger tip 416 areabout 2.0 mm wide, which is about the same width as the conventionalgrounding spring's single-jointed spring finger. Central finger portion406 is preferably about 5.0 mm wide, which is much wider than the widthof the conventional grounding spring's single-jointed spring finger. Asdiscussed below, this wider stance makes the double-jointed springfinger in accordance with the preferred embodiments of the presentinventions less fragile and more robust and stable than the conventionalgrounding spring's single-jointed spring finger.

FIG. 5 illustrates, in perspective view, a portion of heat sink assembly300 and a portion of a PCB 510 where concave finger tip 416 of onedouble-jointed spring finger 304 engages PCB 510. As mentioned earlierwith respect to FIG. 3, double-jointed spring finger 304 extends formgrounding spring 302, which is attached to peripheral surface 314 ofheat sink 310. As shown in FIG. 5, the final installed location ofconcave finger tip 416 of each of the double-joined spring fingers 304relative to PCB 510 will generally fall along a line 512 (i.e., themovement of concave finger tip 416 is substantially limited to what iscommonly referred to as the “z-axis” during compression). Accordingly,the final installed location of the tip can be precisely controlled evenwhen the grounding spring must accommodate a wide variety of installedheights of the heat sink relative to the PCB. This compares favorablywith single-jointed spring finger 114 of conventional grounding spring110 shown in FIG. 2. As shown in FIG. 2, the final installed location ofthe tip of single-jointed spring finger 114 relative to PCB 210 willgenerally fall along an arc 212 (i.e., the length of single-jointedspring finger 114 forms the radius of the arc). This makes the finalinstalled location of the tip of a conventional grounding spring'ssingle-jointed spring finger difficult to precisely control when theconventional grounding spring must accommodate a wide variety ofinstalled heights of the heat sink relative to the PCB.

Grounding springs in accordance with the preferred embodiments of thepresent invention exhibit several other advantages over conventionalgrounding springs. For example, a conventional grounding spring's springfingers, which are fragile because their length is long relative totheir width, can be damaged during handling of the heat sink assembly.Grounding springs in accordance with the preferred embodiments of thepresent invention are less susceptible to being damaged during handlingof the heat sink assembly because the double-jointed spring fingers arenot as fragile as the single-jointed spring fingers. For example, thenarrowest portions of double-jointed spring fingers (i.e., distal fingerportion 410 and concave finger tip 416) are shorter relative to theirwidth as compared to the length of single-jointed spring fingersrelative to their width. This also makes the tip of the double-jointedspring finger less susceptible to sliding off the conductive pads on thePCB. The relatively long length of a conventional grounding spring'ssingle-jointed spring fingers makes them susceptible to sliding off theconductive pads, both during and subsequent to the card assemblyprocess. For example, a conventional grounding spring's single-jointedspring fingers may slide off the conductive pads in the field subsequentto the card assembly process as the result of being subjected to shockor vibration, or as the result of being bumped by a human finger duringmaintenance. In this regard, the preferred embodiments of the presentinvention incorporate concave finger tip 416 that makes it even lesssusceptible to sliding off the conductive pads of the PCB.

FIG. 6 illustrates, in a side view, a circuit card assembly 600 having agrounding spring 302 with double-jointed spring fingers 304 interposedbetween a heat sink 310 and a PCB 605 in accordance with the preferredembodiments of the present invention. Only two double-jointed springfingers 304 are shown in FIG. 6 for the sake of clarity. A bare diemodule 610 includes a module substrate 615, an electronic component suchas a semiconductor chip 620, and an electronic connection 625.Electronic connection, which electrically connects PCB 605 to modulesubstrate 615, may be a pin grid array (PGA), a ceramic column gridarray (CCGA), a land grid array (LGA), or the like. Semiconductor chip620 is thermally connected with heat sink 310 through a thermalinterface 635, which is preferably a layer of thermally conductiveadhesive that also secures heat sink assembly 300 relative to bare diemodule 610 and PCB 605. PCB 605 includes conductive pads, such as solderpads 640, connected to a ground plane (not shown) in PCB 605. Concavefinger tips 416 of double-jointed spring fingers 304 engage solder pads640, preferably in a “mating” ball-in-socket fashion to reduce thelikelihood of concave finger tips 416 sliding off of solder pads 640.Solder pads 640 are located along the side edges of bare die module 610.Preferably, a backside bolster plate 645 is positioned adjacent PCB 605with an insulating layer (not shown) interposed therebetween.

FIG. 7 is a flow diagram of a method 700 for grounding a heat sink inthermal contact with an electronic module using a grounding spring withmultiple-jointed spring fingers according to the preferred embodimentsof the present invention. Method 700 sets forth the preferred order ofthe steps. It must be understood, however, that the various steps mayoccur at any time relative to one another. An electronic module issoldered to a PCB (step 710). A grounding spring having a plurality ofmultiple-jointed spring fingers is attached to a heat sink to form aheat sink assembly (step 720). A thermally conductive adhesive isdispensed on the module (step 730). Finally, the heat sink assembly isattached to the module, with concave finger tips of the multiple-jointedspring fingers engaging conductive pads on the surface of the PCB (step740).

One skilled in the art will appreciate that many variations are possiblewithin the scope of the present invention. For example, the heat sinkmay be mounted by utilizing another attachment mechanism in lieu of athermally conductive adhesive. Moreover, the grounding spring may beheld in place between the heat sink and the PCB using another attachmentmechanism in lieu of fastening the grounding spring to the heat sinkwith screws. In addition, other multiple-jointed spring fingers, such astriple-jointed spring fingers, may be utilized in lieu of double-jointedspring fingers. Thus, while the present invention has been particularlyshown and described with reference to preferred embodiments thereof, itwill be understood by those skilled in the art that these and otherchanges in form and detail may be made therein without departing formthe spirit and scope of the present invention.

1. A method for grounding a heat sink in thermal contact with anelectronic module, comprising the steps of: providing a printed circuitboard having an electronic module mounted on a surface thereof;disposing a heat sink over the surface of the printed circuit card andin thermal contact with the module; interposing a grounding springbetween the printed circuit board and the heat sink, wherein thegrounding spring comprises a conductive material having an openingformed in a generally planar base portion thereof through which the heatsink and the module are in thermal contact with each other, wherein thebase portion of the grounding spring makes electrical contact with atleast a portion of a peripheral surface of the heat sink, and wherein aplurality of multiple-jointed spring fingers extend from the baseportion of the grounding spring to form a plurality of generallyZ-shaped spring elements that make electrical contact with conductivepads on the surface of the printed circuit board, wherein each of themultiple-jointed spring fingers is configured so that movement of aconcave tip thereof is substantially limited to a z-axis as themultiple-jointed spring finger is compressed toward the base portion ofthe grounding spring, and wherein the concave tip makes electricalcontact with one of the conductive pads by receiving in ball-in-socketfashion at least a portion of a conductive pad on the surface of theprinted circuit board.
 2. The method as recited in claim 1, wherein theinterposing step includes a step of attaching the grounding spring tothe peripheral surface of the heat sink using a plurality of screws. 3.The method as recited in claim 2, wherein the step of attaching thegrounding spring to the peripheral surface of the heat sink using aplurality of screws is performed before the heat sink is brought intothermal contact with the module and before the generally Z-shaped springelements are brought into electrical contact with the conductive pads onthe surface of the printed circuit board.
 4. The method as recited inclaim 3, wherein the step of attaching the grounding spring to theperipheral surface of the heat sink using a plurality of screws isperformed by passing the screws through a plurality of through-holes inthe generally planar base portion of the grounding spring and threadingthe screws into a plurality of threaded-holes in the peripheral surfaceof the heat sink.